1. Field of the Invention
The present invention relates generally to an optical disk device, and more particularly, to an optical disk device that reads and plays back data recorded on an optical disk.
2. Description of the Related Art
In order to increase the speed with which data is played back in optical disk devices for CD-ROM and other optical disks, the speed with which the optical disk rotates has been increased. In some cases, the rotation speed of the optical disk is increased some 20 times a standard rotation speed. At such high rotation speeds, however, vibration of the optical disk caused by eccentricity and unbalancing of the optical disk also increases by the square of the speed of angular rotation. This vibration causes the focus servo and the tracking servo to become misaligned and impedes reading of the data.
In a conventional optical disk device, when a data read command is received from an upstream device such as a personal computer the optical disk device reads data from an optical disk at a maximum playback speed. However, if an error is experienced in the reading of the data from the optical disk, then the data playback speed is decreased in order to re-read the data from the disk. After the data is successfully re-read from the optical disk the data read speed is then returned to the maximum playback speed.
The above-mentioned errors may be of several types, among them (1) a CIRC (cross interleaved Reed-Solomon code) error in the data block on the optical disk that the optical disk device is trying to read, the error being uncorrectable; (2) the synchronization of the data block that the optical disk device is trying to read cannot be acquired; and (3) the optical disk device seeks the sector containing the block that the optical disk device is trying to read, but the seek is unsuccessful.
It should be noted that repeated issuance of data read commands from the upstream device personal computer indicates an occurrence of one or more of the above-described types of errors. Accordingly, there is a consequent substantial lengthening of the overall data read time because time is required to detect an error, to decrease the data playback speed and to return the data playback speed to its previous level.
FIG. 6 and FIG. 7 illustrate the above-described condition. In FIG. 6, wherein each command is represented by a pulse and is continuously repeated and executed as shown in (A), the read data is transferred at the rate of one transfer per pulse as shown in (B) and the rotation speed of the spindle motor SPM that rotatably drives the optical disk is approximately uniform as shown in (C).
By contrast, when a data read command is repeatedly issued with respect to a defective sector of the optical disk as shown in FIG. 7, the defective sector data read command execution time lengthens substantially as indicated by the large pulse width as shown in (A), the read data is transferred at the rate of one transfer per pulse as shown in (B) and the rotation speed of the spindle motor SPM that rotatably drives the optical disk fluctuates widely as shown in (C). The wide fluctuation in the speed of the spindle motor SPM reflects a decrease in the data playback speed at defective sectors of the optical disk followed by an increase in the data playback speed after the data is eventually and successfully read from the optical disk. As a result, as described above there is a consequent substantial lengthening of the overall data read time because time is required to detect an error, to decrease the data playback speed and to return the data playback speed to its previous level.
In general, when attempting to execute an optical disk program, a personal computer reads data from the optical disk in units of several tens to several hundreds of blocks at a time at a minimum. However, the data read command is divided among units of several blocks and issued continuously to the optical disk device. Accordingly, with sectors for which re-reading is occurring continuously, the data playback speed is decreased by the data read command and, despite having only just completed reading the data, the next data read command immediately increases the data playback speed in order to read additional data.
If, for example, a sector of an optical disk exists which cannot be read at 20.times. data playback speed, then the data playback speed is decreased and the time taken to complete reading the data from the sector may be several seconds. Accordingly, when a personal computer attempts to read several tens to several hundreds of blocks of data at a time, the time required to read the data when there is an error is several tens of times longer compared to the time required to read the data when there is no error and, as a result, the performance of the optical disk device can appear to have deteriorated by a factor of several tens.